1. Field of the Invention
The present invention generally relates to a distance measurement apparatus and, more particularly, to an active distance measurement apparatus applied in distance detection used in an apparatus (e.g., a camera) requiring distance measurement.
2. Description of the Related Art
FIG. 23 is an example of a conventional active distance measurement apparatus using a PSD (optical position sensing device) 5. In this distance measurement apparatus, pulsed light from a light source 2 such as an IRED (infrared-emitting diode) is projected on an object 1 serving as a distance measurement target, and the position of an incident spot formed on the PSD 5 by pulsed light from the light source 2 upon reflection on the object 1 is detected to measure an object distance.
Referring to FIG. 23, an infrared-ray projected from the light source or IRED 2 through a projection lens 3 is reflected by the object 1 and is incident on the PSD 5 through light-receiving lens 4. An incident position x of the infrared-ray is defined by the following equation (1) when an intersection between the optical axis of the light-receiving lens 4 and the PSD 5 is defined as an origin. ##EQU1## where S is the distance between the projection lens 3 and the light-receiving lens 4, l is the distance between the projection lens 3 and the object 1, and f is the distance between the light-receiving lens 4 and the PSD 5.
The PSD 5 generates a signal photocurrent i.sub.p and outputs signal currents i.sub.A and i.sub.B depending on the incident position x from two output terminals 5A and 5B in accordance with its operational principle. The signal current i.sub.A is defined as follows: ##EQU2## where a is the distance from the PSD end of the IRED 2 side to the origin, and t is the length of the PSD 5.
Equation (4) is derived from equation (3) as follows: ##EQU3## Therefore, a ratio i.sub.A /(i.sub.A +i.sub.B) based on equation 4 is calculated to obtain the distance l.
In addition to the pulsed light from the light source 2, external light is also incident on the PSD 5. A photocurrent component produced by the reflected light of the pulsed light is superposed on an ordinary photocurrent produced by the external light. A detection circuit connected to the output of the PSD 5 must have a function of separating and extracting only the photocurrent component produced by the reflected light of the pulsed light.
This ordinary light extraction method is disclosed in Published Unexamined Japanese Patent Application No. 1-240812, and an ordinary photocurrent extraction technique can be derived therefrom.
The ordinary photocurrent extraction technique and an output depending on the distance l in use of the detection circuit shown in FIG. 23 will be described below.
The output current i (i.sub.A) from the PSD 5 is amplified by a preamplifier 7A and an amplifying transistor 8A through an input terminal 6A. The amplifying transistor 8A is connected to a current source 9A, a current source 10A, a compression diode 11A, and a buffer 12A. The inverting input terminal (-) of a hold amplifier 13A is connected to the amplifying transistor 8A, the noninverting input terminal (+) of the hold amplifier 13A is connected to a compression diode 14A and a current source 15A, and the output terminal of the hold amplifier 13A is connected to a hold transistor 16A, a hold resistor 17A, and a hold capacitor 18A, as shown in FIG. 23.
A differential pair of transistors 21 and 22 constituting a ratio calculating circuit 20 together with a current source 19 receive an output from the compression diode 11A through the buffer 12A. The compression diodes 11A and 14A have same characteristics, and the current sources 9A and 15A are set at the same current level. An output voltage of the hold amplifier 13A is determined such that output voltages of the compression diodes 11A and 14A, i.e., voltages input to the inverting and noninverting input terminals of the hold amplifier 13A are set equal to each other.
In the detection circuit shown in FIG. 23, constituting elements represented by reference numerals 6B to 18B are identical to those represented by reference numerals 6A to 18A and are arranged in the same manner as those of the elements represented by reference numerals 6A to 18A. The suffix A is replaced with B, and a detailed description of the arrangements and operations of these elements will be omitted.
The operation of the detection circuit having the above arrangement will be described below.
An ordinary light storage operation for storing an ordinary photocurrent component is performed. Referring to FIG. 23, the hold amplifier 13A and the current source 10A are turned on in response to a hold signal T.sub.1, and all the circuits except for the IRED 2 and the ratio calculating circuit 20 start to operate. In this state, the output current i of the PSD 5 is equal to an ordinary photocurrent I.sub.p, and the hold amplifier 13A operates to store it in the hold capacitor 18A.
When the ordinary photocurrent I.sub.p is amplified by the preamplifier 7A, the output voltage of the compression diode 11A is decreased by this current, and the potential at the inverting input terminal of the hold amplifier 13A is also decreased. The output voltage of the hold amplifier 13A is increased, and the base potential at the hold transistor 16A is increased accordingly. The ordinary photocurrent I.sub.p is grounded as a collector current through the hold resistor 17A. That is, the ordinary photocurrent I.sub.p is always grounded by a feedback operation without being amplified.
The bias current of the amplifying transistor 8A is supplied from the current source 9A. During non-emission of the IRED 2, no current flows to points A and B in FIG. 23. This ordinary light storage operation continues for a predetermined period of time in accordance with the hold signal T.sub.1, and the charge corresponding to the ordinary light component is stored and held in the hold capacitor 18A. The ordinary light storage operation time is called a stable time.
When the IRED 2 emits light in accordance with an emission signal T.sub.2, the function of the hold amplifier 13A is interrupted by the hold signal T.sub.1. At this time, the ordinary light extraction charge is held in the hold capacitor 18A. While the ordinary photocurrent I.sub.p is being extracted, the signal current i is amplified by the preamplifier 7A and the amplifying transistor 8A, and a signal current flows through the compression diode 11A. During emission of the IRED 2, the current source 10A is turned off in response to the hold signal T1.
An output voltage V of the compression diode 11A is defined by the following equation: ##EQU4## where V.sub.T : thermal voltage
I.sub.S : reverse saturation current PA1 K: gains of the preamplifier 7A and amplifying transistor 8A PA1 ln: Napierian logarithm
As described above, the output current from the PSD 5 is a voltage compressed by the compression diode. The ratio calculating circuit 20 for obtaining the ratio i.sub.A /(i.sub.A +i.sub.B) using this compressed signal will be described below.
Currents I.sub.out and I.sub.z, and voltages V.sub.A and V.sub.B in FIG. 23 are represented by equations (6) and (7) below. ##EQU5## therefore, equation (8) is established as follows: ##EQU6## since I.sub.z =I.sub.o -I.sub.out from equation (6), equations (9), (10), and (11) are obtained as follows: ##EQU7##
A substitution of equation (4) into equation (11) yields equation (12) below: ##EQU8##
As described above, the distance measurement arithmetic output depending on the distance l can be obtained.
As described above, the conventional ordinary light extraction circuit requires the hold capacitor 18A.
First, when this hold capacitor is used, however, the charge voltage leaks over time. For example, when a relatively long period of time is required to perform distance measurement a plurality of number of times upon holding of the ordinary photocurrent component, the extraction current is gradually decreased to result in a distance measurement error. This typically occurs in a bright place with external light.
Second, the capacitor itself has a hold error due to its dielectric absorption. An error current .DELTA..sub.IP is also superposed by the dielectric absorption, and the distance measurement precision is adversely affected.
Third, at present, a tantalum capacitor having a capacitance of about 0.47 .mu.F to 1 .mu.F is used as a hold capacitor in view of hold characteristics. The tantalum capacitor is large in size and expensive when its application to the field of cameras is taken into consideration.
In addition, since the tantalum capacitor is externally attached to an IC chip, extra pins must be arranged in this IC, posing problems on cost and space factor.
The distance measurement arithmetic output thus obtained is represented by a single characteristic line L1 proportional to the reciprocal of the object distance, as shown in FIG. 24. However, the signal light as a photocurrent from the object is normally as very weak as about several 10 pA when the object is remote from the camera, an uncertainty region No represented by a hatched region and surrounded by curves L2 and L3 shown in FIG. 25 is formed due to shot noise of the photocurrent detection circuit and the photocurrent itself, and noise of the sensor itself.
One of the effective means for reducing the uncertainty region No and improving the distance measurement precision is a technique for causing an integral capacitor to store distance measurement arithmetic output currents obtained by the measurement performed a plurality of number of times (n times), as disclosed in each of Published Unexamined Japanese Patent Application No. 1-224617, GB 2212688, and U.S. Pat. No. 5,136,148. This technique is known as a means for extracting a weak signal mixed with noise with a high S/N ratio. According to this means, the uncertainty can be reduced to 1/.sqroot.n.
The above means is implemented for so-called signal processing applicable to all active distance measurement apparatuses and is very useful to improve distance measurement precision. However, this means has the following four problems.
&lt;1&gt;An external capacitor attached to a chip is required to perform accumulation processing.
&lt;2&gt;IC pins for externally attaching the capacitor to a distance measurement IC are required, the distance measurement IC package becomes large in size, and the compactness of the camera is impaired.
&lt;3&gt;Processing circuits such as a circuit for charging/discharging the capacitor and a circuit for resetting the capacitor are required to result in a bulky distance measurement IC circuit.
&lt;4&gt;The external accumulation capacitor must be selected as a capacitor having a small leakage current and small dielectric absorption in the same manner as the hold capacitor. A capacitor which satisfies these requirements is expensive and larger in outer appearance than the normal ceramic capacitor by almost two-fold, thus posing mounting problems. An influence of the dielectric absorption on distance measurement will be described below.
FIG. 26 is an equivalent circuit diagram of this type of external capacitor. When a voltage V.sub.ref is applied to a capacitor C represented in this equivalent circuit through a switch SW, as shown in FIG. 27A, a terminal voltage in the switch-OFF operation is lower by .DELTA.V caused by the dielectric absorption than that in the switch-ON operation, as shown in FIG. 27B.
Assume that three distance measurement operations are performed within a short period of time after the distance measurement IC is powered on. In the first distance measurement operation, when the distance measurement IC is powered on, the integral capacitor is charged to the voltage V.sub.ref. In this case, the voltage V.sub.ref is selected to be a small voltage of about 0.2V so as to minimize the influence of the dielectric absorption. However, in the initial period, as shown in FIG. 27B, the voltage is decreased by .DELTA.V, i.e., several mV, due to the influence of the dielectric absorption. In addition, since the integral capacitor is also susceptible to the influence of dielectric absorption during integration, the charge is absorbed by a capacitor Cs shown in FIG. 26, thereby slightly decreasing the integral voltage.
From the second distance measurement operation, the amount of charge absorbed by the capacitor Cs is reduced, and the distance measurement value in the first distance measurement operation is smaller than that of each of the second and third distance measurement operations by about 10%. In addition, since the dielectric absorption characteristics nonlinearly vary depending on temperatures, it is difficult to propose an effective correcting means.